Spleen cells from mice immunized with human rhabdomyosarcoma cells were fused with X63 cells. The hybridomas secreted antibodies which specifically interacted with human sarcoma cell extracts but not with extracts from normal cells. These antibodies also cross-reacted with cell extracts from other human tumor cells. These findings show that antigenic similarities exist between different tumor cells of human origin. Currently studies are being conducted to identify these tumor-specific antigens.